In a Barber type zero-IF receiver circuit, the base band signal is mixed up to an intermediate frequency and a conventional demodulator is used to recover the frequency modulated signal. The output of the demodulator, which can be separated into the desired recovered analog signal and a direct current (DC) offset voltage, is used to drive a data limiter, and in some cases, provide an automatic frequency control (AFC) error signal for the receiver circuit.
The DC offset voltage is a function of the radio frequency (RF) carrier center frequency, first local oscillator frequency and demodulator component values. The manufacturing tolerance and temperature coefficient of the first local oscillator and demodulator components can cause variations in the DC offset voltage such that a fixed data limiter threshold voltage or AFC reference voltage cannot be used. For proper data limiter operation, it is necessary that the data limiter threshold or reference voltage be derived from the demodulator recovered audio output signal so that the effects of variations in the DC offset voltage with the manufacturing tolerance and temperature can be removed.
One problem with deriving the data limiter threshold voltage is that there is a time delay (caused by a time constant in the circuit to remove the effects of modulation of the RF carrier signal) before the threshold voltage is valid. In receivers which are strobed ON and OFF to improve battery life, the time delay in generating a valid data limiter output can require that the receiver be turned ON earlier than necessary, thus degrading battery life. A further description of the problems associated with deriving the data limiter threshold voltage and one method for accomplishing this derivation is disclosed in U.S. Pat. No. 4,929,851. Using one of the known methods for deriving the data limiter threshold voltage provides acceptable data limiter performance(although with a time delay), but the derived threshold voltage is not optimum for use as an AFC reference voltage. This is because the derived data limiter threshold voltage does not accurately represent the deviation of the actual IF center frequency from the desired on-channel IF center frequency.
Accordingly, it is desirable to provide a zero-IF receiver circuit which improves battery life by eliminating the time delay associated with the derivation of the data limiter threshold voltage from the demodulator output signal. In addition, it is desirable to provide an accurate AFC error signal that can be used to control a local oscillator in the circuit, and thus allow the use of a less expensive oscillator crystal. In applications where an accurate AFC error signal is not available, a high stability local oscillator with an expensive crystal is often required.